The present invention relates to an identification tag, in particular but not exclusively to an identification tag which is responsive to interrogation using radio wave radiation.
Identification tags are conventionally in the order of a few cm""s in size and are used in a number of applications, for example personnel wearable tags and anti-theft tags. Such anti-theft tags are often designed to be interrogated by radio radiation, for example using radio wave radiation having a frequency of several hundred kilohertz; the tags are conventionally attached to items of merchandise in retailing premises so that unauthorised passage of the tags past associated interrogating apparatus triggers an alarm.
Personnel wearable tags are often worn by authorised personnel and are used to provide selective access to facilities, for example equipment and buildings. The personnel tags are frequently in the form of magnetic swipe cards which are interrogated by swiping them physically past magnetic sensors.
Antitheft tags are described in a UK patent application no, GB 2306080A, the tags utilising field effect transistor (FET) technology for mixing interrogating radiation at frequencies of f1 and f2 and thereby generating corresponding mixed radiation at frequencies of f1+A f2 for use in detecting presence of the tags.
Moreover, marker tags for use in surveillance systems are described in a European patent application no. EP 0 142 380 A2. The marker tags each comprise a passive distributed resonant inductor-capacitor circuit exhibiting two electrical resonances, the resonances being detectable for determining presence of the tags.
Furthermore, an identification device is described in a U.S. Pat. No. 5,734,332. The device includes an inactive code carrier, for example a standing acoustic wave component, a frequency mixing section and a double antenna input. The device is operable to receive interrogating radiation at two different frequencies and generate two corresponding received signals. The signals are mixed in the frequency mixing section to generate an intermediate signal having a frequency corresponding to a difference in frequency between the received signals. The intermediate signal is coupled to the acoustic wave device which applies a coded modulation to the intermediate signal. The modulated intermediate signal is then recombined with the received signals to provide a composite signal for emission from the device. Emitted radiation corresponding to the composite signal is detected by equipment interrogating the device, thereby establish presence and identity of the device.
Conventional identification tags suffer from one or more of the following problems:
(a) they do not provide a sufficiently unique response in some applications; this can result in unauthorised personnel gaining access to buildings and equipment;
(b) in anti-theft applications, interrogation equipment associated with the tags are triggered spuriously by objects providing a similar response to that provided by the tags; and
(c) it is sometimes difficult to uniquely identify tags adapted to be interrogated by non-contact interrogation methods, for example using radio wave radiation; and
(d) it is not straightforward to modify identification codes recorded on the tags, for example where the identification tags are affixed to items of merchandise as they proceed through their manufacturing processes, the tags being used for recordal of completed manufacturing processes applied to the items or for quality control purposes.
There is thus a problem of providing each card with an associated unique signature whilst also making it susceptible to non-contact interrogation methods and unsusceptible to counterfeiting.
The inventor has appreciated that it is desirable to improve the reliability of conventional identification tags and associated interrogation equipment by arranging for the equipment to emit a specific form of interrogating radiation and for the tags to be selectively responsive to the radiation. Moreover, the inventor has appreciated that it would be desirable to improve the conventional tags so that information can be recorded on them more easily.
Therefore, according to the present invention, there is provided an identification tag comprising:
(a) transponding means for receiving interrogating radiation and for emitting modulated radiation in response; and
(b) modulating means for modulating the received interrogating radiation in a manner indicative of tag identity to generate tile modulated radiation, the transponding means including first and second structures for receiving first and second radiation components of the interrogating radiation and generating corresponding first and second signals respectively, said first and second components being at mutually different frequencies,
the modulating means including a coupling structure operative to receive the second signal and generate corresponding acoustic radiation and to couple the acoustic radiation to the first structure, the acoustic radiation modulating the first signal in the first structure to generate the modulated radiation, the coupling structure comprising a piezo-electric film for generating the acoustic radiation in response to the second signal.
The invention provides at least one of the following advantages:
(a) the tag is capable of being more reliably identified by a manner in which the modulated radiation is modulated; and
(b) the tag is capable of having its identification characteristics modified by altering the first structure.
In many applications for identification tags, small tag size and low tag cost arm important criteria. Preferably, the coupling structure incorporates a piezo-electric film for generating the acoustic radiation in response to the second signal. This provides the advantage of being a potentially cheap and compact manner to implement the coupling structure.
Acoustic radiation is defined as being mechanical vibrational radiation propagating within a medium.
Advantageously, the film can be of polyvinyl difluoride (PVDF) material which provides a benefit of being mechanically robust.
Conveniently, the tag is operative to be interrogated by radio radiation in a range of frequencies from around 50 MHz to 3 GHz. Thus, the first and second structures can compose radio antennae for receiving the interrogating radiation and emitting the modulated radiation. This provides the advantage that the tag is capable of being physically compact when designed to respond to radiation at this frequency range.
In a first embodiment of the invention, the first structure can incorporate a circular electrode region and the second structure can include a loop antenna. Conveniently, the first structure is spatially within the second structure. This provides the advantage of being a potentially compact arrangement for receiving the interrogating radiation and emitting the modulated radiation.
In a second embodiment of the invention, the first structure can comprise a plurality of electrically interconnected and radially disposed elongate conducting segment tracks, each track operative to function as an acoustic resonator having an associated modal vibrational response and to modulate the first component with the modal response corresponding to a segment track stimulated into resonance by the acoustic vibrations, thereby providing the tag with a signature code modulated onto the modulated radiation. This provides the advantage that the modulated radiation is capable of being modulated by a complex signature by which the tag can be more reliably identified.
In a third embodiment of the invention, the first structure can comprise a launcher electrode connected to the second structure, an interdigitated surface acoustic wave structure and a dipole antenna connected to the interdigitated structure, the launcher electrode operative to couple the second signal to generate surface acoustic waves, the interdigitated structure operative to receive the waves which modulate its electrical characteristics, and the dipole antenna is operative to receive the first component and to generate the first signal which the interdigitated structure is arranged to modulate to provide the modulated radiation. This provides the advantage that the interdigitated structure is capable of modulating the modulated radiation with a signature code, the signature code determined by the manner in which surface acoustic waves propagate through the interdigitated structure.